It is often desirable to replace a conventional metal part with a plastic molded part for a number of reasons such as weight savings and ease of assembly.
In the plastic injection molding art, the usual challenges facing a product designer are to design an article having requisite strength for the product application and uniform surface quality for satisfactory appearance, but to avoid excessive weight, material usage and cycle time. A design compromise must often be made between strength and plastic thickness. A relatively thicker plastic section in the article, such as a structural rib, will incur greater weight, material usage, cycle time and induce sink marks and other surface defects due to thermal gradients in the area of the thickened section.
The exterior surface of the part may have to be "Class A" quality and be paintable in an automated line. Any surface degradation due to sink marks, blemishes or other imperfections were unacceptable for commercial standards as described in U.S. Pat. No. 5,344,596.
As described in U.S. Pat. Nos. 5,225,141, 5,069,858, and 5,447,762, it is known in the plastic molding art to use pressurized fluid in conjunction with the plastic molding of articles. The pressurized fluid is typically nitrogen gas which is introduced into the mold cavity at or near the completion of the plastic injection. The pressurized fluid serves several purposes. First, it allows the article so formed to have hollow interior portions which correspond to weight and material savings. Second, the pressurized fluid within the mold cavity applies outward pressure to force the plastic against the mold surfaces while the article solidifies. Third, the cycle time is reduced as the gas migrates through the most fluent inner volume of the plastic and replaces the plastic in these areas which would otherwise require an extended cooling cycle. Fourth, the gas pressure pushes the plastic against the mold surfaces, thereby obtaining the maximum coolant effect from the mold.
There have been practical limitations in the adaptation of gas injection in the plastic molding field. For example, oftentimes the gas channel will not be hidden as viewed from the "Class A" surface, especially where the "Class A" surface is concave or planar with the gas channel immediately behind the non-convex "Class A" surface.